Re: Do we really need SLOB nowdays?

[Vlastimil Babka <vbabka@suse.cz>]

On 12/10/21 13:06, Christoph Lameter wrote:
> On Fri, 10 Dec 2021, Hyeonggon Yoo wrote:
> 
>> > > (But I still have doubt if we can run linux on machines like that.)
>> >
>> > I sent you a series of articles about making Linux run in 1MB.
>>
>> After some time playing with the size of kernel,
>> I was able to run linux in 6.6MiB of RAM. and the SLOB used
>> around 300KiB of memory.
> 
> What is the minimal size you need for SLUB?
 
Good question. Meanwhile I tried to compare Slab: in /proc/meminfo on a virtme run:
virtme-run --mods=auto --kdir /home/vbabka/wrk/linux/ --memory 2G,slots=2,maxmem=4G --qemu-opts --smp 4

Got ~30800kB with SLOB, 34500kB with SLUB without DEBUG and PERCPU_PARTIAL.
Then did a quick and dirty patch (below) to never load c->slab in
___slab_alloc() and got to 32200kB. Fiddling with
slub_min_order/slub_max_order didn't actually help, probably due to causing
more internal fragmentation.

So that's relatively close, but on a really small system the difference can
be possibly more prominent. Also my test doesn't account for text/data or
percpu usage differences.

diff --git a/mm/slub.c b/mm/slub.c
index 68aa112e469b..fd9c853971d1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3054,6 +3054,8 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		 */
 		goto return_single;
 
+	goto return_single;
+
 retry_load_slab:
 
 	local_lock_irqsave(&s->cpu_slab->lock, flags);